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jokulhlaup

The eruption of Eyjafjallajokull (which means Island Mountain Glacier in Icelandic) started out in March as a relatively quiet and tourist-friendly Hawaiian style eruption. That petered out and then a few days later, the magma reemerged subglacially, producing the spectacular ash-producing phreato-magmatic eruption that has transfixed the world and stranded all would-be European air passengers. The Boston Globe’s Big Picture coverage has been as-usual spectacular. Check out these two photo sets (15 April, 19 April). NASA’s Earth Observatory has also been producing some nice images of the ash plume. Follow them here.

And few days into the eruption, the magmatic heat produced enough glacier melt not just to fuel the ash production but also to generate an outburst flood, as captured on video below:
[youtube=http://www.youtube.com/watch?v=9sryalI57oo&feature=player_embedded]

The video above includes a rather spectacular train of standing waves, which led to some debate amongst some friends and I over whether they represented sub- or super-critical flow. Fortunately, I have access to an expert on the subject, Gordon Grant, who weighed in thusly:

The standing waves represent what is best referred to as “trans-critical flow”, that is flow that oscillates around Froude No. = 1. We have real-time (though very small scale compared to what you’re seeing in the video) measurements from Grant (1997; available on the WPG website). Basically flow on the downstream portion of the wave is accelerating (Fr >1), while flow on the upstream portion is decelerating. The flow then oscillates between Fr > 1 and Fr < 1, maintaining overall flow at close to Fr ~ 1. Cross-sectionally averaged Froude Number tends to be slightly less, due to drag at the boundary. See paper for details.

For the best scientific coverage of the on-going Icelandic eruption, you absolutely can’t miss Erik Klemetti’s Eruptions blog. Erik is a volcanologist and has been doing an astounding job of keeping up with this (and all other volcanic activity). He also benefits from an active, engaged, and informed community of commenters to keep the rest of us up-to-the-minute on volcanic activity around the world.